Field of the Invention
The present invention relates to a microwave oven having its internal chamber walls comprised of a precoated sheet steel (PCM sheet steel) of the type in which a chromate resin mixed paint or anticorrosion molybdenum pigment mixed paint is applied as a primer.
Description of the Prior Art
PCM sheet steel requires no post-working painting, and this provides for simplification of the process for product fabrication and eliminates the need for a painting apparatus for post-forming painting. Because of this fact, PCM sheet steel has been receiving considerable attention as a material for use in constructing the internal chamber walls of microwave ovens. Microwave ovens in which PCM sheet steel is used as internal chamber walls of the oven are known including, for example, one described in Japanese Patent Publication No. 3-61094. The PCM sheet steel used in this prior art type of microwave oven has its primer and top coats formed of a polyester resin.
A microwave oven will cause vapor to be generated within the internal chamber thereof upon heating a foodstuff. In this case, if the interior of the internal chamber is filled with vapor, the food in the internal chamber may not be seen from the outside. In order to increase the commercial attractiveness of the microwave oven and/or to enable a user to determine the progress of cooking in the internal chamber of the oven, it is required that the condition of cooking within the internal chamber be clearly visible to the user from the outside. A known type of microwave oven is designed to meet this requirement, being such that the internal chamber of the microwave oven has intake and exhaust vents provided in the walls thereof so that when vapor is produced within the internal chamber the vapor may be exhausted outward in order to enable the food in the internal chamber to be seen clearly from the outside. The intake and exhaust vents generally consist of punch holes each of the order of 3 to 3.5 mm in diameter.
The PCM sheet steel used in the prior art microwave oven has a drawback that its corrosion resistance is rather low because the sheet steel has its primer and top coats formed of a polyester resin. Where the PCM sheet steel is formed with such punch holes as noted above, it is required that the punched-out portion be subjected to post-coating with a resin paint in order to provide improved corrosion resistance. Technically, however, it has hitherto been impracticable to use a paint having good rust-preventing characteristics in view of possible unfavorable effect of such paint upon the hygienic quality of the oven interior.